The present invention relates to bookbinding apparatus that apply adhesive to a side edge of a series of sheets aligned to form a bundle and bind a cover sheet to the sheet bundle, or bookbinding apparatus in a bookbinding system that produces booklets by sequentially stacking sheets printed at an image forming apparatus to form a bundle of sheets, applying adhesive to a side edge of sheet bundle, and binding a cover sheet to the sheet bundle. More particularly the present invention relates to apparatus improvements that enable uniform application of adhesive to a backside of a sheet bundle, and accurate binding of the cover sheet to the sheet bundle.
Generally, this kind of bookbinding apparatus is widely used as a terminal device of an image forming apparatuses such as printers or copiers. After sheets formed with images are stacked in page order and aligned to form a bundle, the bookbinding apparatus applies adhesive to an edge of the sheet bundle and binds a cover sheet to that to form a booklet. Particularly, there have been a variety of systems proposed that automatically bind and cover a sheet bundle simultaneously to the printing of predetermined information. These systems are becoming increasingly common with the increase in on-demand printing, particularly with the recent improvements in the so-called computer-aided publishing.
This kind of bookbinding apparatus is a device that is separate to an image forming apparatus, and can be used as a stand-alone apparatus that applies adhesive to a side edge (a back side) of printed sheets stacked in a bundle, and forms a booklet by binding a separately conveyed cover sheet to an edge of the sheet bundle. Such bookbinding apparatus are also used as a part of a larger a system by connecting to a discharge outlet of an image forming apparatus. The system forms booklets by sequentially receiving printed sheets conveyed therefrom. For example, unexamined Patent Pub. 1 proposes a system that automatically finishes a booklet of sheets output from an image forming apparatus. According to that publication, sheets output by an image forming apparatus are received from a discharge outlet of that device and are guided to the discharge path. Then, they are stacked in a tray provided at a downstream side of the discharge path. The sheet bundle stacked on the tray in a substantially horizontal posture is turned 90 degrees and conveyed in a vertical posture to an adhesive applicator to be glued. The apparatus folds a cover sheet supplied from an inserter provided at a discharge path over the glued sheet bundle thereby binding the cover sheet to the sheet bundle to form a booklet.
In the system disclosed in Patent Publication, a container holding adhesive is provided to apply adhesive to a sheet bundle held in a vertical posture by gripping means. A roller is disposed in the container. The roller is coated with adhesive and travels along the bottom edge of the sheets to apply adhesive thereto. Hot-melt adhesive is used as the adhesive or bonding agent. The adhesive is in a solid form when charged to the container, making it is easier to handle. The adhesive is melted in the container, and impregnates the roller so that it can be applied to the edge of the sheet bundle. After application to the sheet bundle edge, the adhesive hardens and adheres to the sheets.
It is necessary to adjust the thickness of the adhesive layer when applying adhesive to an edge of a sheet bundle. For example, if the sheet bundle is thick, a thick layer of adhesive will be securely infused between the pages of the sheets in the bundle thereby ensuring a good bond. Conversely, if the sheet bundle is thin and the adhesive layer is thinner, the adhesive will not be pressed out to the front and back surfaces of the cover sheet when it is folded over the sheet bundle. It is known practice in the art to adjust the layer of adhesive applied by adjusting the gap between the applicator roller and sheet bundle edge, and to use a blade, plate or the like, to scrape or sweep away excess adhesive. Conventionally, a blade is disposed perpendicularly to the edge across the back of the sheet bundle. As it moves from one end of the sheet bundle to the other, the blade forms a uniform adhesive layer.    Patent Publication: Japanese Unexamined Patent Publication No. 2004-209869
To form a uniform adhesive layer when applying adhesive with an applicator roller to the backside edge of the sheet bundle held in the predetermined posture as described above, a knife edge of the plate-shaped blade is used to scrape or sweep away adhesive. Conventionally, the plate-shaped blade is disposed across the backside of the sheet bundle perpendicular to the edge of the sheet bundle. For that reason, the adhesive layer is unevenly formed by the applicator roller. If the viscosity of the adhesive is not uniform, the blade cause blemishes when sweeping away excess adhesive, thus it is impossible to form a uniform adhesive layer with this type of system. For example, as shown in FIG. 10, because of the surface tension, the adhesive will droop from both edges causing the center area to develop channels. Channels will remain at the center area if the blade (represented by the line z-z in that drawing) sweeps away adhesive to the thickness (h in the drawing) of the adhesive layer. If the viscosity of the adhesive is high when swept away, deficiencies such as blemishes will remain.
When a cover sheet is folded over the sheet bundle, either the unevenness formed by the adhesive layer appears in portions of the backside of the booklet, or wrinkles will appear. If the viscosity of the adhesive is lowered in order to solve these problems, the adhesive will droop and make it difficult to completely solve the problem of channels being formed. Also, the adhesive layer is thinner at the edges of the sheet bundle than the center because of the later cover sheet binding process but adhesive does not bleed out to the cover sheet or backside sheet surfaces when the cover sheet is folded over the sheet bundle. It is impossible to form such an adhesive layer with the conventional blade structure.
The present invention is based on forming an even layer of adhesive while pushing away excess adhesive when forming a predetermined layer of adhesive using a blade. The present invention provides an adhesive applicator that forms adhesive to a smooth surface at a predetermined thickness when it is applied to a sheet bundle edge. There is neither unevenness nor wrinkles in the backside cover sheet when binding the cover sheet and the esthetic appearance of the booklet is maintained. Furthermore, the present invention provides a bookbinding apparatus that can provide a cosmetically acceptable cover when binding a cover sheet to form a booklet, by applying adhesive to the edges of a sheet bundle of varying thicknesses.